Bentley ft al

ABSTRACT

FORCE MEASURING SYSTEM HAVING A SENSING INDUCTANCE AND A MOVABLE FORCE SENSITIVE ELEMENT FOR CHANGING THE EFFECTIVE Q OF SAID INDUCTANCE. A SERVO RESTORING COIL FOR RESTORING THE MOVEMENT OF SAID MOVABLE ELEMENT IS PROVIDED. AN OSCILLATOR HAVING A TRANSISTOR WITH AN OUTPUT TERMINAL IS ALSO PROVIDED AND INCLUDES A LOADING CIRCUIT HAVING SAID INDUCTANCE COUPLED TO THE OUTPUT TERMINAL AND ARRANGED TO VARY THE EFFECTIVE LOADED WITH CHANGES IN THE POSITION OF SAID MOVABLE ELEMENT. FEEDBACK MEANS IS ARRANGED TO CAUSE VARIATION IN SAID LOAD TO CHANGE THE AMPLITUDE OF OSCILLATION AND MEANS IS PROVIDED TO APPLY THE CHANGES IN AMPLITUDE TO SAID SERVO RESTORING COIL TO OPPOSE THE MOTION OF SAID MOVABLE FORCE SENSITIVE ELEMENT.

June 27, 1972 o. z. BENTLEY HA FORCE MEASURING SYSTEM 2 Sheets-Sheet 1Original Filed Jan. 13, 1958 N wmSm lnvembru June 27, 1972 E. BENTLEYETAL Re. 27,411

FORCE MEASURING SYSTEM Original Filed Jan. 13, 1958 2 Sheets-Sheet 2 97'l/IOE 99 10// -95 FIGUBE 3 mreflf'ors DOA/A40 E. 5E-nY [67 EU 4. P2 /CE165 9' Q 6 United States Patent Office Re. 27,411 Reissued June 27, 197227,411 FORCE MEASURING SYSTEM Donald E. Bentley, Minden, Nev., andRobert L. Price, Thousand Oaks, Califl, assignors to Systron-DonnerCorporation, Concord, Calif.

Original No. 3,057,195, dated Oct. 9, 1962, Ser. No. 708,433, Jan. 13,1958. Application for reissue Apr. 24, 1970, Ser. No. 31,606

Int. Cl. G01l1/08 US. Cl. 73-141 R 14 Claims Matter enclosed in heavybrackets [II appears in the original patent but forms no part of thisreissue specification; matter printed ln italics indicates the additionsmade by reissue.

ABSTRACT OF THE DISCLOSURE Force measuring system having a sensinginductance and a movable force sensitive element for changing theeffective Q of said inductance. A servo restoring coil for restoring themovement of said movable element is provided. An oscillator having atransistor with an output terminal is also provided and includes aloading circuit having said inductance coupled to the output terminaland arranged to vary the effective load with changes in the position ofsaid movable element. Feedback means is arranged to cause said variationin said load to change the amplitude of oscillation and means isprovided to apply the changes in amplitude to said servo restoring coilto oppose the motion of said movable force sensitive element.

This invention relates to force measuring systems and more particularlyto force measuring systems of the servo type, employing sensing elementswhich vary the parameters of oscillatory circuits.

Force measuring systems of various types employing oscillatory circuitsare well known. In the usual circuit, an input force is caused to varythe tuning or frequency of an oscillator and the resulting shift infrequency detected in various ways to effect the measurement. In othercircuits, the input force is caused to vary both frequency and amplitudeor amplitude alone. The variation is usually accomplished by changingtuning elements and detection is accomplished in the various methodsknown to the art.

Such prior art circuits generally have endeavored to cope with problemsrelative to stability, sensitivity and susceptibility to inputs otherthan the force to be measured. Such disabilities often arise from thecircuit design values necessary to obtain the required sensitivity rangeor other features. The high frequencies often necessary have beenanother limiting factor. Many prior art systems have been sensitive tovibration, subject to micro- 5 phonics, undamped for desired ranges ofoperating frequencies, sensitive to physical positioning, and so on.Outputs have generally been of a pulse type and have required specialcircuitry to yield the desired measurement information. It is an objectof the invention herein to overcome in large measure these disabilitiesand limitations.

The invention herein relates to a force measuring system employing anoscillatory circuit wherein the force to be measured causes a change inthe effective Q of the load inductance. Servo damping is obtainedthrough critical damping by the proper choice of circuit components,without auxiliary circuitry. In the invention, a movable vane of amaterial having a high resistivity per unit area is caused to reflectvarying amounts of impedence into the plate or collector circuit of anoscillator and thereby affect the amplitude of oscillatory output. Theamplitude variations thus created are detected, and, if desirable in theparticular application, amplified and the output of the detector oramplifier employed to restrain the motion of the force sensing element.This output also yields measurement information. Relatively lowfrequencies are employed and low power levels are used in the sensingcircuits. The variation in spacing necessary to obtain substantialoutput is very slight.

In the invention, considerable latitude is possible in the design of theoscillator itself. Various systems may be used, within certain designconsiderations. These considerations include requirements that theoscillatory circuit include an inductance in the plate or collectorcircuit, and include component values preferably such that theoscillator is normally operating near its threshold of oscillation, sothat a variation in tank Q gives a substantial variation in the level ofthe output.

It is an object of the invention to provide a force measuring system ofgreat sensitivity and stability, which is relatively insensitive toinputs other than the force to be measured. It is also an object of theinvention to provide a force measuring system employing simple andstraightforward circuit and design.

It is an object of the invention to provide a force measuring systemwherein the movable sensitive force sensing element does not affectfrequency, and which consequently may readily be separated from theoscillatory portions of the circuit, giving flexibility of design andallowing miniaturization of the sensing element if desired. In thisconnection, the low frequencies and low power levels employed are ofmaterial benefit.

Further objects of the invention are to provide a servo force measuringsystem which is highly stable and noncritical of adjustment and which istherefore economical of manufacture and operation.

It is also an object of the invention to provide a servo force measuringsystem wherein very slight, practically infinitesimal movements of theforce sensitive element are sufficient to give very reliable indicationof the force being measured notwithstanding the most adverse externaland environmental conditions.

Other objects and advantages of the invention are that servo damping isreadily obtainable through critical damping through the circuitcomponents alone, notwithstanding the fact that servo loop gain may beextreme and that differing systems may have varying moments of inertia.Furthermore, it is an object of the invention that the oscillatorfurnished an output which is DC. and not pulsed, which has lower outputnoise and which therefore has minimum interference with the bandpass ofthe servo loop under varying signal conditions, thereby avoiding thefrequency limitations imposed in the usual pulsed output type ofcircuit.

Another advantage and object of the invention resides in its lack ofsensitivity to microphonics and vibration. The invention imposesvirtually no restraint regarding physical resonances.

A further object of the invention is to provide a force measuring systemwhich is easily stabilized and which has low interaction between circuitcomponents. Other objects and advantages will be understood from thefigures and the detailed description hereinafter following, where FIGURE1 is a schematic diagram of an embodiment of the invention.

FIGURE 2 is a schematic diagram of another embodiment of the invention;and

FIGURE 3 is a perspective view of the main elements of a typical forcesensing system.

In the embodiment of FIGURE 1, force sensing system 11 includes asensing inductance 13, a movable force sensitive element 17 and a servorestoring coil 15. Sensing inductance 13 serves as a part of the plateload circuit for the oscillator comprised of tube 21 and its associatedcomponents. The element 17 is preferably formed of an electricallyconducting, non-magnetic material.

Force sensing system 11 is shown in dotted lines to indicate that it maybe physically located apart from the circuits of FIGURE 1.

The force to be measured causes movement of movable force sensitiveelement 17. This motion causes a change in the effective loading of theoscillator plate circuit due to the influence of movable element 17 onsensing inductance 13. The variation in oscillatory output occasioned bythis change in plate loading is detected, amplified and applied to servorestoring coil 15. Servo restoring coil 15 operates to restrain theoriginal motion or tendency toward motion of movable force sensitiveelement 17 and as a consequence, in the usual arrangement, movable forcesensitive element 17 actually moves very little. An output is taken inthe embodiment shown, across servo restoring coil 15, and this outputgives a direct indication of the force acting on movable element 17.

The oscillator of FIGURE 1 includes a tube 21 having a plate, a grid anda cathode, as shown. The grid of tube 21 is connected to a frequencydetermining tuned circuit, including a capacitance 27 in series with aninductance 25. A grid leak resistance 29 is connected between the gridof tube 21 and ground. The cathode of tube 21 is biased in theconventional manner by resistance 35, which resistance is by-passed bycapacitance 3'7. The plate load of tube 21 includes a resistance 31, anda capacitance 33 in series with a sensing inductance 13. The connectionsto sensing inductance 13 are preferably shielded by a shield 19 to avoidstray pick-up and other interfering signals.

Resistance 39 and capacitance 41 form a de-coupling network isolatingthe plate circuit of tube 21 from the balance of the circuit. Feedbackbetween the plate and grid circuits of tube 21 is accomplished byfeedback capacitance 23 and the plate to grid capacitance of tube 21.The constants of the circuit so far described, and especially the sizeof capacitance 23, are preferably chosen so that the oscillator operatesnear its threshold of oscillation. In this condition, a variation of theeffective load, including sensing inductance 13, will have a greatereffect on the amplitude of oscillatory output, as well appear more fullyhereinafter.

The output of the oscillator is connected, as shown in FIGURE 1, todiode 45, which rectifies the oscillations. Capacitance 47 filters theoutput of diode 45, which output is applied to the grid of tube 53. Tube53, with its associated components, functions as a DC. amplifier,increasing the amplitude of the output of diode 45. Resistance 49 is thegrid leak resistance for tube 53, in addition to functioning as aportion of the load and filter circuit for diode 45. Resistance 57 and59 form a voltage divider network, providing the correct voltage for thescreen grid of tube 53, and resistance 55 is the plate load for tube 53.

The output of the amplifier including tube 53 is applied CIl to a phaseshifting and load network, including capacitances 63 and 69 andresistances 65, 67 and 71.

A cathode follower circuit including tube 73 and its associatecomponents serves to couple the output of the DC. amplifier to servorestoring coil 15. Resistance 75 in the cathode circuit of tube 73serves as the load impedence for the cathode follower, and an outputtaken on the cathode side of resistance 75 is fed through servorestoring coil 15 and resistance 79 to ground. The voltage fiuctuationsdeveloped across resistance 79, which fluctuations are in accordancewith the currents flowing through servo restoring coil 15, provide theoutput of the force measuring system. Resistance 79 may be replaced withother forms of impedance, such as an inductance or a capacitance tocause the circuit to measure forces such as rate of change ofacceleration or velocity, rather than the acceleration or pressuremeasured in the embodiment shown.

In the operation of the circuit of FIGURE 1, the oscillator includingtube 21 is oscillating at a frequency determined primarily by the sizeof capacitance 27 and inductance 25, at a level of oscillation which ispreferably arranged near the threshold of oscillation. The amplitude ofoscillation is determined, inter alia, by the impedance of sensinginductance 13, since the voltage appearing across this inductance willdirectly influence the amount of feedback through capacitance 23 to thegrid circuit of tube 21. This being so, variations in the impedance ofsensing inductance 13 caused by the movements of metallic vane 17 causein turn fluctuations of the oscillatory amplitude. Some change infrequency due to changes of inductance in sensing inductance 13 withvariations in the position of vane 17 may also be present, but any suchchanges in frequency are not detected or of any appreciablesignificance. The load impedance or Q also directly affects output for agiven amplitude of oscillations, adding to the feedback effect. Theloading of the plate circuit impendance is due to the eddy currentlosses in the metallic vane 17.

The output of the oscillator is detected or rectified by diode 45,amplified by the amplifier including tube 53, and applied to the servorestoring coil 15 through the cathode follower including tube 73. Whenthe vane or movable force sensitive element 17 is moved by an externalforce in such a manner as to increase the impedance of sensinginductance 13, an increase in the oscillatory output is rectified,amplified and applied to servo restoring coil 15 which is arranged torestrain the movable element 17 from moving in the direction to causesaid increase in oscillatory output.

A transistorized version of the force measuring system is shown inFIGURE 2. It should be understood throughout this specification and thedescription, that the oscillator, the rectifier, the phase correcting,and the impedance matching system may all be varied in accordance withthe art in the respective fields, and that this invention relates to thecombination of these elements to produce the desired result.

In FIGURE 2, a transistor 95, consisting of an emitter 97, a base 101and a collector 99, together with its associated elements, forms anoscillator similar to the oscillator in FIGURE 1. The collector 99 isconnected to a load circuit including a sensing inductance 97 and acapacitance 117, which are the primary frequency-determining elements ofthe circuit. The emitter circuit includes a resistance 103 and acapacitance 105. Proper operating potentials for the oscillator arefurnished by battery 111.

Resistance 109 and resistance 113 provide proper bias to base 101 oftransistor 95. Capacitance 115 by-passes resistance 113. Capacitanceserves both to by-pass resistance 103 and to shift the phase of thefeedback energy. Capacitance 107 serves as a feedback capacitance fromthe collector 99 to the emitter 97 and is chosen of such a value to justmaintain oscillation when sensing inductance 87 presents minimumimpedancei.e., when the movable force sensitive elements 85 is in itsclosest position to sensing inductance 87. The force sensing system inFIGURE 2 is enclosed in dotted lines and includes sensing inductance 87,movable force sensitive element 85, magnet 89, and servo restoring coil91 operating in conjunction with said magnet 89 to restrain the motionof moveable element 85 when said movable element 85 tends to move inresponse to an input force 83.

Inductance 87 and capacitance 117 are normally tuned to a frequencywithin the usual ratio frequency range. Variations in the position ofmovable element 85 cause inductance 87 to present varying amounts ofimpedance. As the amount of impedance offered by inductance 87increases, greater amounts of collector energy are fed back throughfeedback capacitance 107 to emitter 97, and the level of oscillations isincreased. There may be some frequency change with these variations inthe particular embodiment shown in FIGURE 2, but this frequency chargenot used or detected, and may be disregarded. The voltage provided bybattery 111 is normally within the range of from 1 to 60 volts.

The output of the oscillator is detected or rectified by diode 121, andthe DC output appears across capacitance 123, and across the networkincluding resistances 125, 127, 131 and 133 in series. Capacitance 129is in parallel with resistance 127. Resistances 125, 127, 131 and 133are arranged to provide the proper DC. bias to the bases of transistors135 and 137. Diode 121 may be reversed, and resistance 133 in such acase is connected to battery 143 instead of battery 111 in order toreverse the polarity. Transistors 135 and 137 are arranged asdirect-coupled cascaded amplifiers, and in the configuration shown inFIGURE 2, handle the signals in the positive direction. Transistors 139and 141 are direct-coupled cascaded amplifiers, and in the configurationshown in FIGURE 2, handle the signals in the negative direction.Transistors 135, 137, 139 and 141 connected as shown comprise a DC.coupled complementary symmetrical amplifier.

Resistance 133 is chosen of a high enough value to hold the voltage atresistance 131 to nearly zero. Resistance 131 is normally very small toprovide a slight forward bias for each of the two sets of transistors135 and 137, and 139 and 141.

Capacitance 129 serves to provide a phase lead to frequencies in therange of the servo cut-off in order to prevent the system as a wholefrom oscillation, and also to insure a good damping factor. The outputfrom transistors 135, 137, 139 and 141 is fed directly through the servorestoring coinl 91 and through the terminating impedance 147.

By proper choice of the terminating impedance 147 the force measuringsystem of FIGURE 2 may be made to serve a number of different uses. Forexample, when output impedance 147 is a resistance, the system serves asan accelerometer, or with an appropriate transducer, pressure, weight,rate of flow, etc. If the output impedance 147 is inductive, the systemserves to measure the rate of change of acceleration or pressure, etc.,and if the output impedance is capacitative the system may be employedas a velocity meter.

In the operation of the circuit of FIGURE 2, an input force 83 is causedto act on movable element 85. The change in position of movable element85 with respect to sensing inductance 87 causes a change in theimpedance of the collector load circuit. Moveable element 85 has theeffect of a shorted turn on sensing inductance 87. The change inimpedance of sensing inductance 87 causes, in turn, a change in thelevel of oscillations, and this change in the output level is rectifiedby diode 121. The output of diode 121, in turn, is applied to theincludes transistors 135, 137, 139 and 141. The output of thesetransistor is fed through servo restoring coil 91 which reacts, inconjunction with magnet 89, to restrain the movement of movable element85 which was induced by input force 83.

The circuit is normally arranged so that the output from the symmetricalamplifier is just enough to balance the input force. The total movementof the movable force sensing element is normally less than a fewthousandths of an inch over the entire range of measurements.

In FIGURE 3 a typical force sensing system is shown to be comprised ofmagnets and 157 arranged so that lines of flux link servo restoring coil153 wound on core 151. The ends of servo restoring coil 153 are broughtout to terminals 169 and 171. Atfixed to core 151 by a shaft 159 is ametallic paddle 161. The sensing inductance 163 is pancake wound toprovide good sensitivity to the positioning of paddle 161. The ends ofsensing inductance 163 are brough out to terminals and 167.

In the operation of the embodiment of FIGURES 3, a forcefor example, anacceleration-tends to cause movement of movable element 151. If theacceleration is to be left, movement element 161 tends to move closer tosensing inductance 163. This motion lowers the effective Q of inductance163 and through the circuit of an amplifier and a rectifier, such asthat shown in FIG- URE l or FIGURE 2, is caused to change the current inservo restoring coil 153, thereby preventing the movement of paddle 161except by a very small amount.

It will be apparent to those skilled in the art that various departuresmay be made from the invention described herein without departing fromthe spirit and scope thereof. The invention, therefore, is to be limitedonly by the claims that follow.

What is claimed is:

1. A force measuring system comprising: a force sensing system includinga sensing inductance, a movable force sensitive element for changing theeffective Q of said inductance, and a servo restoring coil for restoringthe movement of said movable element; an oscillator having a transistorwith an output terminal and including a loading circuit having saidinductance coupled to the output terminal and arranged to vary theeffective load with changes in the position of said movable element andfeedback means arranged to cause said variation in said load to changethe amplitude of oscillation; and means to apply said changes inamplitude to said servo restoring coil to oppose the motion of saidmovable force sensitive element.

2. In a force balance system, a metallic vane coupled to the force to bemeasured, a pancake-wound inductance positioned adjacent to saidmetallic vane so that its effective impedance is changed by variationsin the position of said metallic vane, a radius frequency oscillatorhaving a transistor with an output terminal and including saidinductance as a part of its resonant circuit with the inductance coupledto the output terminal of the transister, and having electricalconstants such that changes in said impedance result in variations inthe output amplitude of said oscillator, a detector for rectifying saidvariations in output, an amplifier for amplifying said rectified output,a servo restoring coil for restraining the motion of said metallic vane,and impedance matching means coupling the amplifier output to said servorestoring coil.

3. In a force measuring system, a radio frequency oscillator having atuned circuit including a sensing inductance, the constants of the tunedcircuit being chosen so that the oscillator operates substantially as aClass A oscillator, a movable element formed of conducting nonmagneticmaterial positioned adjacent the sensing inductance and being adapted tointersect the lines of force from the sensing inductance and to couplelosses with the sensing inductance to thereby change the Q of the tunedcircuit, means for rectifying the output of the oscillator, a servorestoring coil for controlling movement of said element, and impedancematching means connecting the restoring coil to the rectifying means [Jsaid impedance matching means including first and second power supplylines for supplying power to said impedance matching means, first andsecond active devices, means interconnecting said first and secondactive devices and connecting said first active device to said firstpower supply line and said second active device to said second powersupply line and the outputs of said first and second active devices tosaid restoring coil whereby the means for rectifying can operate bothdevices to supply output signals of either polarity to the restoringcoil.

4. A force measuring system as in claim 3 together with a D.-C.amplifier connected between the rectifying means and the impedancematching means.

5. In a force measuring system, a radio frequency oscillator having atransistor with an output terminal and a tuned circuit including an aircore inductance [,1 coupled to the output terminal of the transistor, amovable element of conducting non-magnetic material positioned adjacentthe inductance and being adapted to intersect the lines of force fromthe inductance to thereby change the Q of the inductance, the oscillatorbeing arranged so that changes in Q of the inductance result inmodulation of the amplitude of the output of the oscillator, meansrectifying the output of the oscillator, a servo restoring coil forrestraining movement of said element, [and] impedance matching meansconnecting said rectifying means to said servo restoring coil L] andphase lead means in series with the rectifying means and the impedancematching means for damping the system.

6. A force measuring system as in claim 3 [wherein said oscillatorincludes a vacuum tube having plate, grid and cathode elements, andwherein the amplitude of the output from the plate is modulated]together with phase lead means in series with the means for rectifyingand the impedance matching means tor damping the system.

7. A force measuring system as in claim 3 wherein said oscillatorincludes a transistor having collector, emitter and base elements, andwherein the amplitude of the output from the collector is modulated.

8. In a force measuring system, a radio frequency oscillator having atransistor with an output terminal of the transistor and a tuned circuitincluding an air core inductance [,1 coupled to the output terminal ofthe transistor, a movable element of conducting non-magnetic materialpositioned adjacent the inductance and being adapted to intersect thelines of force from the inductance to thereby change the Q of theinductance, the element being movable from a normal position away fromand towards the inductance, the constants of the tuned circuit beingchosen so as to achieve unity loop gain without saturation with theelement in its substantially normal position so that as the element ismoved from its normal position the loop gain is modulated, meansrectifying the output of the oscillator, a servo restoring coil forrestraining movement of the element, and impedance matching meansconnecting the output of the rectifying means to the servo restoringcoil.

9. A force measuring system as in claim 8 together with a D.-C.amplifier connected between the rectifying means and the servo restoringcoil.

10. A servo type force balance system including: an oscillator operatingnear its threshold of oscillation and having a transistor with an outputterminal and a load circuit including an inductnce coupled to the outputterminal of the transistor, a metallic vane positioned near saidinductance and having a tendency to move in accordance with the forceapplied to it, the movement of said vane inducing variable losses intothe inductance to thereby vary the output of the oscillator, restrainingmeans connected to said vane and coupled to the output of the oscillatorto apply a restraining force to the vane to balance the force applied tothe vane, and means coupled to the output of the oscillator andindicating the force tending to move the vane.

11. A force measuring system comprising an oscillator arranged tooscillate near its threshold of oscillation at a radio frequency, saidoscillator including a transistor with an output terminal and a resonantcircuit having an air core inductance coupled to the output terminal, ametallic vane of conducting non-magnetic material positioned to causechanges in the effective Q of said resonant circuit with changes in itsposition due to its effect on said inductance to thereby causevariations in the output of the oscillator, the force to be measuredtending to shift the position of the vane, means coupled to the outputof the oscillator to detect variations in the output of the oscillator,a servo restoring coil connected to the metallic vane and coupled to thedetecting means, means for establishing a magnetic field in the vicinityof the servo restoring coil, the detecting means producing a currentflow in the servo restoring coil proportional to the force applied tothe vane so that the servo restoring coil produces a restoring force tothe vane to balance the force tending to shift the position of the vane,and an output impedance connected in series with the servo restoringcoil so that the current flowing in the restoring coil also flows in theoutput impedance and produces an output signal across the outputimpedance indicating the force tending to shift the position of thevane.

12. A force measuring system including a transistor having an emitter, abase and a collector, a resonant circuit connected to said collector,said resonant circuit including an inductance and a capacitance, amovable metallic vane positioned near said inductance to affect theimpedance thereof to thereby change the effective Q of said resonantcircuit with a change in the position of the vane with respect to theinductance, the force being measured tending to shift the position ofthe vane, feedback means connected between said collector and saidemitter of a size to just cause oscillations at a predetermined spacingbetween the vane and the inductance, means to detect said oscillations,means forming a magnetic field, and a servo restoring coil connected tosaid vane and coupled to the output of the detecting means, said senvorestoring coil being disposed in said magnetic field and yieldably restraining said vane against substantial movement by the force tending tomove said vane.

13. A force measuring system comprising: magnetic means creating linesof flux, a servo restoring coil arranged to link said lines of flux, ametallic vane coupled to said servo restoring coil, the vane having atendency to move in response to the force being measured, a sensinginductance positioned adjacent to said vane, an external circuit linkingsaid sensing inductance to said restoring coil, said external circuitincluding an oscillator having a transistor with a collector and havinga resonant circuit with said sensing inductance forming a part of theresonant circuit [,1 and being coupled to said collector, saidoscillator also having a feedback coupling of such a size thatvariations in the effective Q of the resonant circuit are induced in theresonant circuit by changes in position of the vane relative to theinductance to thereby cause variations in the output of the oscillator,said oscillator being arranged to oscillate near its threshold ofoscillation at a radio frequency, means coupling said variations in theoutput of the oscillator to said servo restoring coil to cause currentflow in said servo restoring coil and to thereby create a restoringforce on the vane which balances the force tending to move the vane, andmeans connected to the coupling means to indicate the force tending tomove the vane.

14. In a force measuring system, a radio frequency oscillator having atransistor with a collector and having a tuned circuit including asensing inductance [,1 coupled to the collector, the constants of thetuned circuit being chosen so that the oscillator operates substantiallyas a class A oscillator, a movable element formed of a conductingnou-magnetic material positioned adjacent the sensing inductance andbeing adapted to intersect the lines of force from the sensinginductance to thereby modify the Q of the inductance, the changing ofthe Q of the inductance causing modulation of the amplitude of the radiofrequency oscillations, the amplitude of the radio frequencyoscillations being controlled by the position of the movable elementwith respect to the inductance, means for rectifying the modulatedoutput of the oscillator, a servo restoring coil connected to themovable element and coupled to the rectifying means, and means forming amagnetic field in the vicinity of the restoring coil so that currentflow in the restoring coil applies a restorig force to the movableelement to balance the force applied to the movable element.

References Cited 10 UNITED STATES PATENTS 2,593,339 4/1952. Ostermann eta1. 73-141 R UX 2,614,163 10/1952 Roper 73-398 R UX 2,790,145 4/1957Bartelink 73-194 R X 2,847,625 8/1958 Popowsky 73-398 R X 2,849,6698/1958 Kinkel 73-141 R X US. Cl. X.R.

2233 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.Re. 27, 411 Dated June 27, 1972 Inventor) Donald E. Bentley and RobertL. Price It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

Column 5, line 49, "coi-nl" should be coil--.

Column 7, Claim- 8, lines 38, 39, cancel "of the transistor".

Column 9, Claim 14, line 8, cancel "storig" and substitute therefor -storing.

Signed and sealed th1s,,l8th day of June 19714,.

(SEAL) Atteat:

EDWARD M.F'LETCHER,JR.- C. MARSHALL DANN Attesting Officer Comisaionerof Patents

